Essentially that large-scale, complicated alliances can result in small scale wars unexpectedly spiraling out of control. In the case of WWI there were multiple small conflagrations in the Balkans before WWI but it then took just the right one to set it off. In a similar vein, one wouldn't be surprised if one the similar small conflagrations around Russia like are currently happening leads to a Russia v. NATO war with little warning. Similarly, one could expect a similar situation in the Pacific given the many border conflicts there.